Logic systems of the single ended cascode voltage switch (CVS) type, also called cascode current switch (CCS) logic, are described in the commonly assigned U.S. Pat. No. 4,591,993, to W. R. Griffin and L. G. Heller. These circuits are generally made in the complementary metal oxide semiconductor (CMOS) technology having an N channel transistor, matrix as a logic system using a P channel transistor matrix as a complementary logic network.
The cascode voltage switch type logic system may also be of a differential type as disclosed in commonly assigned U.S. Pat. No. 4,638,482, to J. W. Davis and N. G. Thoma. However, in differential CVS circuits, each CVS logic function has two outputs, True and Complement, and therefore uses twice the AC power and twice the wiring channels that single-ended CVS logic would use.
FIG. 1 shows a state of the art CMOS single-ended CVS type decoder. There are two approaches to providing an up level or logical "1" at the decoder outputs. The first approach, shown in FIG. 2, is to use a passive pull-up device, or load resistor to pull unselected outputs high. There are two problems with this approach. First, the decoder is slow. The passive load must be small enough for a branch of the CVS tree to pull it to a valid low. Therefore, the decoder outputs' rise time is limited by the load current, which is determined by the number and size of the devices stacked in the branch, i.e., the beta ratio of the tree branch to the load. Second, there is DC current flow in the selected output, so there is excessive DC power dissipation.
The second approach, shown in FIG. 3, is to pre-charge the outputs to Vdd by clocking an active device, turn the clocked devices off which floats the outputs, and then either enable the CVS tree or enable the decoded inputs so the selected output is pulled low. This approach also has two problems. First, it requires clocks for timing. Second, since unselected nodes float during decode this clocked, or dynamic, CVS tree is unusable in radiation hardened applications. Device count is 46.
The present invention overcomes the above limitations of prior CVS type circuits.